STDOUT(4FREEBSD) - man page online | special files

NAME

fd, stdin, stdout, stderr — file descriptor files

DESCRIPTION

The files /dev/fd/0 through /dev/fd/# refer to file descriptors which can be accessed
through the file system. If the file descriptor is open and the mode the file is being
opened with is a subset of the mode of the existing descriptor, the call:
fd = open("/dev/fd/0", mode);
and the call:
fd = fcntl(0, F_DUPFD, 0);
are equivalent.
Opening the files /dev/stdin, /dev/stdout and /dev/stderr is equivalent to the following
calls:
fd = fcntl(STDIN_FILENO, F_DUPFD, 0);
fd = fcntl(STDOUT_FILENO, F_DUPFD, 0);
fd = fcntl(STDERR_FILENO, F_DUPFD, 0);
Flags to the open(2) call other than O_RDONLY, O_WRONLY and O_RDWR are ignored.

IMPLEMENTATION NOTES

By default, /dev/fd is provided by devfs(5), which provides nodes for the first three file
descriptors. Some sites may require nodes for additional file descriptors; these can be
made available by mounting fdescfs(5) on /dev/fd.